A. Field of the Invention
The present invention relates generally to the field of healing bone fractures. More particularly, it concerns apparatus and methods for healing bone fractures by controlling the load transmitted to the fractures throughout the healing process.
B. Description of the Related Art
Spinal columns are prone to deformation and degeneration. These conditions often require corrective surgery. In some surgical procedures it is necessary to remove the intervertebral discs from the damaged part of the spinal column. Bone grafts may then be placed in the position vacated by the disc and the bone graft and vertebrae to be fused are held in a desired position for healing using a device connected to vertebrae on either side of the bone graft. Such a device maintain the vertebrae in a desired position for healing and further prevents the transmission of load through the bone graft while the graft is healing. Devices for the connection of vertebral bodies are disclosed in U.S. Pat. Nos. 4,611,581 and 4836196.
In other procedures the healing of fractured long bones employs implantable plates for internal fixation. The concept is to place a plate in contact with the fractured bone such that the plate spans the fracture, and to fasten the plate to the bone on both sides of the fracture. Compression plates typically are attached to the bone by screws and nuts that act to transfer the load from the bone to the plate and force the broken ends of the bone together. Typical fracture fixation compression devices are disclosed in U.S. Pat. Nos. 4,943,292, 4,513,744, 3,779,240, and 3,552,389.
In a typical situation, use of a conventional bone fracture compression plate made of high modulus 316L stainless steel (200 GPa; 30.times.10.sup.6 psi), may result in stress-shielding the bone under the plate causing a weakening of the cortical bone (at about 15 GPa) under the plate. Relieving the bone from carrying a load over an extended period of time may thus contribute to the development of this type of bone weakening or osteoporosis, also known as osteopenia. When the plate and screws are removed from the healed bone, a re-fracture may result due to such weakening.
Attempts to solve this problem have included the fabrication of the fracture plates from materials that are less rigid than 316L stainless steel. For example, titanium alloys, composites, and resorbable materials have been utilized. Each of these materials, however, presents additional problems.
Bioresorbable bone plates have been investigated by Christal et al.,(1984). A major problem associated with resorbable bone plates made of polyglycolic acid (PGA) or polylactic acid (PLA) polymers is the release of polymer residues into the body. While it is unclear whether these polymer residues produce any side effects, the precise control of the dissolution rate of these polymers over the entire plate is difficult, if not impossible, to control. Moreover, the screws for securing the bone plate to the bone cannot be made from these materials because these materials lack the requisite torsional strength. Hence, conventional metal screws are used, and a secondary operation is needed to remove the screws even when the plates are made of resorbable materials.
One of these conventional compression plate assemblies (U.S. Pat. No. 5,057,111 to Park) utilizes a spacer placed between the screw and the nut to allow the plate to move slightly, resulting in the load being transmitted differentially between the compression plate assembly and the bone. As the bone fracture heals, a greater load is transmitted to the bone. The spacer is made of a material which plastically deforms as a function of time and load transmitted therethrough to decrease the load transmitted between the structure and the fastener. This movement of the fracture plate relative to the bone is known as creep.
The deposit of polymer residue near the site of the wound may be deleterious to the healing process if the spacer breaks down. Also. there is little control over the rate of creep due to changes in the polymer. Thus, here exists a need for a nut for use in securing a bone plate to a bone that allows creep while controlling the unwanted escape of polymer particles.